The invention relates to a method for recovering precious metal components by leaching gold and/or silver out of ores or ore-concentrates and also out of precious-metal wastes or precious-metal scrap. Method for recovery of the precious metal values employs cyanide-containing leaching solutions having a pH value between 7 and 13 by adding an aqueous H.sub.2 O.sub.2 solution.
In the heretofore conventional industrial procedure for leaching precious metals out of ores or ore-concentrates or other precious metal containing substances, the starting material is finely ground and dispersed in water to form a slurry. The pH value of the ore slurry is then raised into the alkaline pH range by adding lime or milk of lime, whereupon the complexforming cyanide is added, as a rule in the form of NaCN in an aqueous solution of approximately 5 to 20%. Next the ore slurry is agitated, with introduction of air, for 24 to 48 hours in stirring tanks or Pachuca tanks for a time period of 24 to 48 hours.
Because cyanides hydrolyze in the acid pH range while forming the highly toxic hydrocyanic acid, the pH value of the ore slurry must be raised into the alkaline range and be kept there. Ordinarily, the process takes place in the pH range from 10 to 11. Typically, this pH range is set by adding lime and milk of lime in the ratio of 80:20.
The cyanide concentration of the ore slurry depending on the ore composition ranges from 0.025 to 0.25 % by weight of NaCN. The cyanide is metered in the form of an aqueous solution of 2 to 20 % into the first of up to ten cascaded leaching tanks traversed by the slurry.
With regard to the introduction of air into the leaching slurry, a concentration of dissolved oxygen in the slurry is chosen to be as close as possible to the physically determined maximum value of about 8 to 9 ppm because at the practicable conventional cyanide concentration of 0.3 to 3 kg/ton of ore, the diffusion of the dissolved oxygen in the slurry determines the rate of leaching.
The leaching of gold and silver in cyanide solutions when using NaCN takes place according the formula below: ##STR1##
The above reaction takes place in two steps: ##STR2##
The peroxide formed in the first reaction step because of its decomposition into oxygen and water increases the concentration of the dissolved oxygen in the slurry. In lieu of the NaCN shown herein, KCN also may be used just as well.
It is known to use hydrogen peroxide when recovering precious metals by means of cyanide-containing leaching solutions. One process does not use the hydrogen peroxide to oxidize the precious metals, instead it uses it to detoxify the leaching slurries before they are drained away. The cyanide present in the slurries is decomposed therein by addition of hydrogen peroxide into non-toxic cyanate and ammonia. The decomposition of the cyanide takes place relatively rapidly only in the pH range of the known leaching procedure initially cited, wherein the oxygen is inserted into the leaching slurry in the form of blown air.
Another known procedure (US patent 3,826,723) provides for adding an aqueous H.sub.2 O.sub.2 solution to preciousmetal leaches by means of cyanide-containing leaching solutions. This procedure, however, is not economical on account of its unusually high consumption of cyanide and would not be implemented on a commercial scale. In this known procedure, a shorter leaching time requires the cyanide concentration to be between 1 to 600 g of NaCN per liter of leach. In an illustrative example thereof 60 g of NaCN per liter of leach and 5 ml of stabilized 50 % by weight H.sub.hd 2 O.sub.2 are added per liter of leach. Moreover the known procedure requires adding 1 g of lignine sulfate per liter of leaching solution.
The molar ratio of NaCN/H.sub.2 O.sub.2 set in the above known procedure indicates that operation takes place at a pH value between 9.5 and 11.5 as in the previously described detoxification of the leaching slurries. Because of the marked decomposition of the NaCN in its reaction with the added H.sub.2 O.sub.2 only that NaCN added in excess is available for complexing the precious metals.
German patent 36 37 082, describes a procedure for the leaching of gold and/or silver out of ores or oreconcentrates employing an aqueous cyanide leaching solution with a pH of 8 to 13 and adding an aqueous solution of H.sub.2 O.sub.2. However, this prior process is carried out by controlling the addition of said H.sub.2 O.sub.2 solution by means of the concentration of the oxygen dissolved in the leaching solution, the oxygen content in said solution being set at 2 to 20 mg/liter. The purpose thereof is to carry out the leaching more 0 economically, in particular by reducing the H.sub.2 O.sub.2 consumption, without thereby entailing higher sodium cyanide consumption relative to the previously known leaching with air blowing.